(1) Field of the Invention
This invention is generally related to the location of an acoustic source in a fluid medium and more specifically to a method and apparatus for locating such a remote acoustic source by range and depth in the medium.
(2) Description of the Prior Art
Over the past several years diverse methods and apparatus have been developed for locating a remote acoustic source by its range and depth, particularly in the ocean. Examples of these systems are described in Bates, S. M. and Bilazarian, P., "The Comparison, Benchmarking, and Application To Source Localization of Low Frequency Underwater Acoustic Propagation Models" I.E.E.E. Oceans '90 Proco, 209-215 (1990).
Certain of these methods and apparatus depend upon a geometrical analysis. For example, multiple sensors at different locations can establish horizontal bearings or vertical angles called depression/elevation angles. Also a single sensor can be moved over time to a different location to establish bearings and depression/elevation angles. An intersection of those bearings or depression/elevation angles is taken as the location of the acoustic source. Such systems generally use simplified ocean acoustic models. For example, when the sound-speed is assumed to be constant over the entire ocean, acoustic rays tend to follow cyclical paths that can intersect at different locations or convergent zones. Thus, although the convergence of the rays at one zone can be taken as the location of an acoustic source, the specific location has some ambiguity with respect to other convergence zones.
Other methods and apparatus utilize a sensor array to receive the acoustic signals and produce an acoustic field distribution at the array location. Then a system uses a forward propagation analysis by effectively positioning a hypothetical source at successive ranges and depths within each range throughout a predetermined geographical area. At each range and depth, the signals from the source are propagated to the array. The sampled and propagated distributions are compared at each successive source location. The acoustic source is taken to be at a location from which the forward propagated acoustic field most closely matches the measured field. Such systems, while relatively accurate, are expensive to implement since the number of possible source locations is extremely large.
Bates, S. M. and Bates, B. J., "Source Localization By Inversion of The Parabolic Equation Method", J. Acoust. Soc. Am. 85,S18 (1989) suggest the localization of an acoustic source by backpropagating measurements from a vertical array utilizing a narrow angle (i.e., less than .+-.20.degree.) parabolic equation solved by a so-called "split-step" method. In accordance with this approach, an initial field is established by measurement at a vertical transducer array. Then the parabolic algorithm is solved by the split-step method for successive incremental ranges expanding from the array. More specifically, the amplitude and phase of the signal are added coherently to the existing field and the result is propagated to the next source range and depth. This analysis over the respective ranges and depths theoretically yields a maximum amplitude at a range and depth corresponding to the location of the acoustic source. Although this approach is more efficient than the previously described prior art approaches, it does not permit the use of all accurate oceanographic databases that define acoustically pertinent variables. Consequently, a method and apparatus based upon the split-step parabolic solution, like the other prior art methods and apparatus severely limits the use of acoustically pertinent variables and introduces some uncertainty in the results. However, with the incorporation of more detailed ocean acoustic models and the addition of an index, initialization, and wide angle capability, the split-step method could be an alternative to the IFD method.
Lee, D. and Botseas, G., "IFD: An Implicit Finite-Difference Computer Model For Solving The Parabolic Equation", NUSC TR659, 1983, is a publicly available paper that describes a wide angle implicit finite difference solution for the wide angle parabolic equation. This solution forward propagates a field from a source and is particularly adapted for incorporating a wide variety of acoustically pertinent variables. However, this solution has not been utilized or suggested for use in the localization of an acoustic source from a remote location. Although it could be used as an alternative to the forward propagation techniques described above, that use would not overcome the inherent problems involved with forward propagation approaches.